The present invention relates generally to a process for evaluating the quality of single crystal silicon. More particularly, the present invention relates to an improved process for detecting the presence of B-defects, formed by the agglomeration of silicon self-interstitial point defects, by means of metal decoration and silicon etching.
Single crystal silicon, which is the starting material for most processes for the fabrication of semiconductor electronic components, is commonly prepared by the so-called Czochralski (xe2x80x9cCzxe2x80x9d) method. In this method, polycrystalline silicon (xe2x80x9cpolysiliconxe2x80x9d) is charged to a crucible and melted, a seed crystal is brought into contact with the molten silicon, and then a single crystal is grown by slow extraction. After formation of a neck is complete, the diameter of the crystal is enlarged by decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached. The cylindrical main body of the crystal which has an approximately constant diameter is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process, but before the crucible is emptied of molten silicon, the crystal diameter must be reduced gradually to form an end-cone. Typically, the end-cone is formed by increasing the crystal pull rate and heat supplied to the crucible. When the diameter becomes small enough, the crystal is then separated from the melt.
In recent years, it has been recognized that a number of defects in single crystal silicon form in the crystal growth chamber as the crystal cools after solidification. Such defects arise, in part, due to the presence of an excess (i.e., a concentration above the solubility limit) of intrinsic point defects, which are known as vacancies and self-interstitials. It is understood that the type and initial concentration of these point defects in the silicon, which become fixed at the time of solidification, are controlled by the conditions under which the single crystal silicon ingot is grown. (See, e.g., PCT/US98/07365 and PCT/US98/07304.) If the concentration of such point defects reaches a level of critical supersaturation within the single crystal silicon, and if the mobility of the point defects is sufficiently high, a reaction, or an agglomeration event, will likely occur.
Vacancy-type defects are recognized to be the origin of such observable crystal defects as D-defects, Flow Pattern Defects (FPDs), Gate Oxide Integrity (GOI) Defects, Crystal Originated Particle (COP) Defects, crystal originated Light Point Defects (LPDs), as well as certain classes of bulk defects observed by infrared light scattering techniques, such as Scanning Infrared Microscopy and Laser Scanning Tomography. Also present in regions of excess vacancies are defects which act as the nuclei for ring oxidation induced stacking faults (OISF). It is speculated that this particular defect is a high temperature nucleated oxygen agglomerate catalyzed by the presence of excess vacancies.
Defects relating to self-interstitials are less well studied. However, it is generally believed that two types of such defects exist, commonly being referred to as A-defects and B-defects (or A- and B- xe2x80x9cswirlsxe2x80x9d or xe2x80x9cclustersxe2x80x9d). A-defects are larger and more easily detected by means common in the art, as compared to B-defects. A-defects are commonly regarded as being low densities of interstitial-type dislocation loops or networks. Less is known about B-defects, primarily because they are much smaller in size and also because, to-date, methods for easily and reliably detecting such defects does not exist. However, at least some believe B-defects are not dislocation loops but rather are loosely packed three-dimensional agglomerates of silicon self-interstitials and impurity atoms of some kind. (See, e.g., F. Shimura, Semiconductor Silicon Crystal Technology, Academic Press, Inc., San Diego Calif. (1989) at pages 282-284 and the references cited therein.) Although A- and B-defects are not believed to be responsible for gate oxide integrity failures, an important wafer performance criterion, A-defects are at least widely recognized to be the cause of other types of device failures usually associated with current leakage problems. B-defects, on the other hand, are currently of less concern, primarily because of their smaller size. However, this is likely to change in the future as integrated circuit manufacturers continue to make smaller devices.
The density of such vacancy and self-interstitial agglomerated defects in Czochralski silicon is conventionally within the range of about 1*103/cm3 to about 1*107/cm3. While these values are relatively low, agglomerated intrinsic point defects are of rapidly increasing importance to device manufacturers because such agglomerated defects can severely impact the yield potential of the material in the production of complex and highly integrated circuits. As a result, accurate and efficient detection of such defects is critical for purposes of both quality assurance and process control.
Historically, agglomerated intrinsic point defects have been detected by processes wherein a sample of the single crystal silicon is chemically treated, and then inspected using an optical microscope or a scanning electron microscope. (See, e.g., F. Shimura at pages 243-271.) For example, the silicon sample may be treated with an etch solution, such as a Secco etch solution, in order to delineate agglomerated defects. Areas of the silicon which contain such defects may appear as xe2x80x9cpitsxe2x80x9d or xe2x80x9chillocksxe2x80x9d on the surface of the sample. The sample is then inspected using optical microscopy, and the pits or hillocks are counted.
As an alternative to the etching process, the quality of a single crystal silicon sample has also been evaluated by chemically treating the sample with copper or some other metal, such as aluminum, nickel, iron or lithium. Typically, copper decoration has involved covering the surface of the sample with copper nitrate and then heating it in an argon atmosphere for 30 minutes at 950xc2x0 C. or more. (See, e.g., F. Shimura at 260.) Like the etching process, the sample is then inspected using optical or infrared microscopy in order to count the defects which are present.
Although the above processes allow for the detection of agglomerated intrinsic point defects, both require the aid of some form of instrumentation in order to detect the defects, after such defects have been delineated by chemical treatment. Such detection processes are time consuming, prohibitively so when used as a means of process control for the production of single crystal silicon. In addition, sample inspection using microscopy typically involves examining only a very small portion of the sample surface. Therefore, this approach often leads to confusion due to inaccurate or inconsistent results because, depending upon what area of the surface is examined, areas where defects are present may be only partially observed or entirely overlooked. Furthermore, existing methods of defect detection are generally inconsistent and unreliable for detecting smaller B-defects; in fact, because of their small size, B-defects are often not detected at all.
In view of the foregoing, a need continues to exist for a process which allows for the more efficient and accurate detection of agglomerated intrinsic point defects in single crystal silicon, and particularly B-defects; a process which may be performed in a sufficiently short period of time, such that single crystal silicon growth conditions may be efficiently monitored to ensure the formation of agglomerated defects are minimized or eliminated.
Among the objects of the invention, therefore, is the provision of a process for more efficiently and reliably detecting the presence of agglomerated intrinsic point defects, particularly defects associated with the agglomeration of silicon self-interstitials, and more particularly B-type agglomerated interstitial defects; the provision of a process for detecting the presence of slip dislocations; the provision of a process for delineating agglomerated intrinsic point defect precipitation zones; the provision of a process for differentiating the precipitation behavior of oxygen precipitates; the provision of a process which allows for the detection of such defects visually under ordinary or enhanced lighting conditions and without the aid of microscopy or other instrumentation, such as laser scattering or X-ray diffraction; the provision of a process for delineating the interface between vacancy dominant regions and silicon self interstitial dominant regions (i.e., the V-I boundary); and, the provision of a process which allows for such visual inspection, thus providing the ability to efficiently identify and distinguish regions within the sample in which the types of intrinsic point defects are different.
Briefly, therefore the present invention is directed to an improved process for delineating B-type agglomerated silicon self-interstitial defects in a single crystal silicon sample. The process comprises heat-treating the single crystal silicon sample at a temperature and for a time sufficient to thermally stabilize B-defects present in the sample, such that these defects are incapable of being dissolved upon a subsequent thermal treatment. The heat-treated sample is then cooled, and a surface of the heat-treated sample is coated with a composition containing a metal which is capable of decorating the stabilized B-defects. The coated sample is then heat-treated at a temperature and for a time sufficient to diffuse the metal into the sample. The heat-treated sample containing the metal is cooled, causing the metal to become critically supersaturated and precipitate at the stabilized B-defect sites. A non-defect delineating etch is performed, followed by a defect delineating etch to reveal the decorated B-defects.
The present invention is further directed to an improved process for revealing slip dislocations in a single crystal silicon sample. The process comprises optionally heat-treating the single crystal silicon sample and cooling the heat-treated sample and then coating a surface of the cooled sample with a composition containing a metal which is capable of decorating slip dislocations. The coated sample is then heat-treated in an inert atmosphere at a temperature and for a time sufficient to diffuse the metal into the sample. The heat-treated sample containing the metal is cooled, causing the metal to become critically supersaturated and precipitate at the slip dislocations. A non-defect delineating etch is performed, followed by a defect delineating etch to reveal the decorated slip dislocations.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.